As a conventional method for manufacturing a glass base material, there are known manufacturing methods including a deposition step of preparing a glass-fine-particle-deposited body by the OVD method, the VAD method, or the like and a transparency-imparting step of heating the glass-fine-particle-deposited body to prepare a transparent glass body (base material) (for example, see Patent Documents 1 to 3).
In the manufacturing method of Patent Document 1, a glass source material gas is introduced into a burner for forming glass fine particles through a pipe under reduced pressure with heating and vaporizing a glass source material, so that pipe temperature is, for example, controlled to 55° C. and hence it becomes possible to use a pipe made of a vinyl chloride-based material having a heat-resistant temperature of 70° C.
In the manufacturing method of Patent Document 2, after a glass source material gas is discarded for a predetermined period of time prior to the start of glass fine particle deposition, the deposition of glass fine particles is started and the discarded amount of the source material gas, volume of the pipe, pressure of the pipe inside, and temperature of the pipe are controlled so as to satisfy a predetermined relationship among them, thereby intending to avoid occurrence of air bubbles and cloudiness in the glass base material. The pipe temperature is controlled to be 82° C. or 85° C.
In the manufacturing method of Patent Document 3, as a method for suppressing unevenness which may occur on the surface of the glass-fine-particle-deposited body, it is described to keep the conduit pipe from a source material gas-generating device that supplies the source material gas to the burner at 90° C. or higher all over the length using a heater and a heat insulating material but there exists no description of temperature gradient of the conduit pipe. Also, there is no description of Reynolds number of the source material gas flowing in the pipe. Moreover, there is no description of particle diameter and aggregation of particles. Furthermore, there is no description of Stokes number of the glass fine particles.
In Patent Document 4, as a method for improving a source material yield, there is described a method of introducing a gas into inner circumference of a hood placed at a tip of the burner flame to suppress the spreading of the flame.
Moreover, as a method for manufacturing a glass-fine-particle-deposited body, there is commonly known a method for manufacturing a glass-fine-particle-deposited body by a vapor-phase synthetic method such as the VAD method, the OVD method, or the MMD method. As for the particle diameter of the glass fine particles, for example, in Patent Document 5 in which a porous soot body obtained by the vapor-phase synthetic method is impregnated with a mixed liquid containing additive fine particles dispersed therein and is heated and subjected to transparency impartment to form a glass base material, it is described to control the particle diameter of an SiO2-based porous body to 500 to 1,000 nm.
Patent Document 6 describes a manufacturing method in which glass fine particles prepared previously are introduced into the burner flame. It is described that the average particle diameter of the glass fine particles to be charged is preferably controlled to 0.2 μm or less.
Moreover, Patent Document 7 describes a manufacturing method in which a glass-fine-particle-molded body is sintered by microwave heating, and it is described that the average particle diameter of the glass fine particles is from 1 nm to 100 μm.